Field of the Invention
The present invention relates generally to programmable implantable pacemakers, and more particularly to an implantable dual chamber pacemaker programmed to operate in an atrial rate based mode, wherein mode switching means are provided for automatically switching the mode of operation of the pacemaker from one mode to another in the event the sensed atrial rate exceeds a prescribed upper rate limit.
Modern programmable pacemakers are generally of two types: (1) single chamber pacemakers, and (2) dual chamber pacemakers In a single chamber pacemaker, the pacemaker provides stimulation pulses to, and/or senses cardiac activity within, a single chamber of the heart, e.g., either the right ventricle or the right atrium. In a dual chamber pacemaker, the pacemaker provides stimulation pulses to, and/or senses cardiac activity within, two chambers of the heart, e.g., both the right ventricle and the right atrium. Typically, only the right atrium and/or the right ventricle is coupled to the pacemaker because of the relative ease with which a pacing lead can be transvenously inserted into either of these chambers. However, the left atrium and left ventricle can also be paced just as effectively providing that suitable electrical contact is made therewith.
In general, both single and dual chamber pacemakers are classified by type according to a three letter code. In this code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a "V" indicating the ventricle, an "A" indicating the atrium, and a "D" indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters to identify the atrium or ventricle or both, and wherein a "0" indicates no sensing takes place.
The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting ("I") response wherein a stimulation pulse is delivered to the designated chamber after a set period of time unless cardiac activity is sensed during that time, in which case the stimulation pulse is inhibited; (2) a Trigger ("T") response wherein a stimulation pulse is delivered to a prescribed chamber of the heart a prescribed period after a sensed event; (3) or a Dual ("D") response wherein both the Inhibiting mode and Trigger mode are evoked, inhibiting in one chamber of the heart and triggering in the other.
Thus, for example, a DVI pacemaker is a pacer (note that throughout this application, the terms "pacemaker" and "pacer" may be used interchangeably) that paces in both chambers of the heart, but only senses in the ventricle, and that operates by inhibiting stimulation pulses when prior ventricular activity is sensed. Because it paces in two chambers, it is considered as a dual chamber pacemaker. A VVI pacer, on the other hand, is a pacer that paces only in the ventricle and senses only in the ventricle. Because only one chamber is involved, it is classified as a single chamber pacemaker. It should be noted that most dual chamber pacemakers can be programmed to operate in a single chamber mode.
Much has been written and described in the art about the various types of pacemakers and the advantages and disadvantages of each. For example, reference is made to U.S. Pat. No. 4,712,555, to Thornander et al., co-invented by the present applicant, wherein some helpful background information about pacemakers and the manner in which they interface with a patient's heart is presented. This patent is hereby incorporated herein by reference.
One of the most versatile programmable pacemakers available today is the DDD pacemaker. This pacer represents a fully automatic pacemaker which is capable of sensing and pacing in both the atrium and ventricle. When functioning properly, the DDD pacer represents the dual chamber pacemaker with the least number of drawbacks. It is typically implanted in patients in an effort to maintain AV synchrony while providing bradycardia support.
In general, DDD pacing has four functional states: (1) P-wave sensing, ventricular pacing (PV); (2) atrial pacing, ventricular pacing (AV); (3) P-wave sensing, R-wave sensing (PR); and (4) atrial pacing, R-wave sensing (AR). Advantageously, for the patient with complete or partial heart block, the PV state of the DDD pacer tracks the atrial rate, which rate is set by the heart's S-A node, and then paces in the ventricle at a rate that follows this atrial rate. Because the rate set by the S-A node represents the rate at which the heart should beat in order to meet the physiologic demands of the body, at least for a heart having a properly functioning S-A node, the rate maintained in the ventricle by such a pacemaker is truly physiologic.
Those skilled in the art have long recognized the advantages of using an atrial rate based pacemaker. For example, U.S. Pat. No. 4,624,260, to Baker, Jr., et al., discloses a microprocessor-controlled dual chamber pacemaker having conditional atrial tracking capability. Similarly, U.S. Pat. No. 4,485,818, to Leckrone et al., discloses a microprocessor-based pacer which may be programmed to operate in one of a plurality of possible operating modes, including an atrial rate tracking mode.
Unfortunately, in some instances, it is possible for a given patient to develop fast atrial rhythms which result from pathological tachycardias and fibrillation. In these cases, the DDD pacer will pace the ventricle in response to the sensed atrial disrhythmia up to the programmed maximum tracking rate. While this upper rate limit is designed into the pacemaker to protect the patient from being paced too fast, it is not desirable to pace at the maximum upper rate limit for a long period of time, else the heart cannot efficiently perform its function of pumping blood through the body.
Therefore, attempts have been made in the art to prevent such atrial arrhythmias from developing. For example, U.S. Pat. No. 4,722,341, to Hedberg et al., teaches an atrium-controlled pacemaker wherein the pacemaker temporarily switches from an atrial rate based mode to a non-atrial rate based mode for a fixed number of stimulation pulses if the sensed atrial activity indicates an atrial arrhythmia may be developing. Unfortunately, however, for some patients, a temporary switching from one mode to another may not be sufficient to correct or arrest the arrhythmia.
What is needed is an atrial rate based pacemaker which will not only sense the atrial arrhythmia once it develops, but which will also take whatever corrective action is needed, for however long (i.e., not just temporarily), to prevent the heart from being paced at the maximum upper limit for long periods of time.
It is known that the dual chamber pacemaker itself may undesirably support (and even induce) some cardiac arrhythmias. This process is described, for example, in U.S. Pat. No. 4,788,980, to Buchanan et al., where such arrhythmias are referred to as a pacer mediated tachycardia, or PMT. The referenced patent discloses a particular technique for recognizing a PMT and terminating it once it develops. Similarly, U.S. Pat. No. 4,712,556, to Baker, proposes another technique for identifying PMT's, and proposes yet another technique for terminating such PMT. Still another patent, U.S. Pat. No. 4,554,921, to Boute et al., teaches modifying the atrial refractory period of the pacemaker in an attempt to break or terminate a PMT.
Regardless of the source of the arrhythmia, however, whether caused by a PMT or by other factors, if left unchecked, the DDD pacer will track the fast atrial rate and pace the ventricles up to the maximum tracking rate for a long period of time, resulting in low cardiac output. What is needed, therefore, is a method or technique for preventing an atrial rate based pacemaker from pacing the heart at the maximum pacing rate for prolonged periods of time, even when an atrial arrhythmia is present.
Sometimes it is possible at the time of implant of a pacemaker to determine whether an atrial fibrillation, atrial flutter, or atrial tachycardia condition is going to develop. In such instances, the pacemaker may always be programmed to operate in a different mode of operation, the leads may be repositioned within the heart, or other actions may be taken to minimize the likelihood of such arrhythmias occurring. Unfortunately, however, it is not always possible at the time of implant to determine whether a patient will develop an arrhythmia as a result of pacing.
Therefore, if such arrhythmias subsequently occur, they must be treated using other techniques, such as through the administration of drugs. Needless to say, the administration of drugs requires the attendance of a physician. Unfortunately, however, a physician is not always present when such arrhythmias develop, and even when a physician is available, such drugs undesirably also suppress the ability of the S-A node to increase the sinus rate during periods of exercise, emotional stress, or other physiologic stress. Thus, the use of such drugs effectively prevents the pacer from functioning as a true physiologic rate-responsive pacer.
What is needed is an approach for dealing with arrhythmias which develop after implant without necessitating the attendance of a physician and without compromising the pacer's ability to function as a physiologic rate-responsive pacer.
It is also possible that the atrial arrhythmia may be caused by the pacemaker's inability to sense P-waves. In such an instance, the paced competition with the native atrial activity may precipitate an atrial tachycardia or fibrillation. This inability to sense P-waves may be caused by numerous factors, but is usually caused by electrode dislodgement or movement, tissue growth, or other events which may occur several days or weeks after implant.
The ability of the pacemaker to sense P-waves is referred to as atrial sensitivity. At implant, the atrial sensitivity is adjusted based on various tests in order to ensure that P-waves are sensed with an adequate margin of safety. However, even this margin of safety may disappear over time, and it thus becomes necessary for a physician to reprogram the atrial sensitivity so that P-waves will be sensed. However, until reprogramming of the atrial sensitivity takes place, there is some possibility that P-waves will not be sensed, resulting in the undesirable atrial arrhythmias described above.
Thus, what is needed is a pacemaker which includes means for periodically checking, and adjusting as required, the atrial sensitivity, thereby assuring that P-waves will always be sensed by the pacemaker. U.S. Pat. No. 4,708,144, to Hamilton et al., represents one approach known in the art for automatically controlling the sensitivity of the pacemaker.
Further, with an atrial rate based dual chamber pacemaker, there is always the problem that a sustained activity period of the patient, resulting in a naturally high sinus rate, may be interpreted by the pacemaker as a pathological atrial arrhythmia. Hence, an atrial rate based pacemaker needs to incorporate some means to readily distinguish a sustained pathological atrial arrhythmia from a sustained activity period, and take appropriate action in each instance.
Advantageously, the pacemaker described herein, including the method of operating such pacemaker, addresses the above and other needs.